CS21:Intro to Computer Science

(Fall 2017)

Course Info | Schedule | Grading | Academic Integrity
Student Support | Tips for Success | Code Style | Piazza | Links
Introduction to Computer Science using Python 3


  • Lab 11 should be done before the final exam
  • Our final exam is Friday, Dec 15, 7-10pm, in Sci Cen 101

Course Information

Section 1: MWF 10:30–11:20, Science Center 256
Professor: Sara Mathieson
Office: Science Center 260
Office hours: Friday 3-5pm
Piazza: CS21 Q&A forum

Links to all sections:

Welcome to CS21. This course will introduce fundamental ideas in computer science while also teaching you how to write computer programs. We will study algorithms for solving problems and implement solutions in the Python programming language. Python is an interpreted language that is known for its ease of use. We also introduce object-oriented programming and data structures. A deeper coverage of these topics will be presented in CS 35.

This course is appropriate for all students who want to learn how to write computer programs and think like computer scientists. It is the usual first course for computer science majors and minors. Students with advanced placement credit or extensive programming experience should place out of this course and instead begin with CS31 or CS35.


book pic We will primarily be using the online book How to think like a computer scientist: Learning with Python by Elkner, Downey and Meyers.

See the Schedule for each week's reading assignment.

Here are a few other useful online resources:

Goals for the course:

By the end of the course, we hope that you will have developed the following skills:


Here is the tentative schedule; it may change as we go...

Sep 04


Introduction to Python and Unix

* Slides 1
* Handout 1

* Slides 2
* Transcript 2

* Slides 3
* welcome.py
* graduate.py

Lab 0: unix, editing

Sep 06


Sep 08


Sep 11


Writing simple programs

* Slides 4
* right_triangle.py
* pretty_print.py
* Transcript 4

* Slides 5
* miles_loop.py
* Transcript 5

* Slides 6
* even_loop.py
* len_loop.py
* sum_loop.py

Lab 1: first programs

Sep 13


Sep 15

Drop/add ends


Sep 18



* Slides 7
* stretch.py
* random_avg.py
* conditionals1.py
* Transcript 7

* Slides 8
* movie.py
* telephone.py

* Slides 9
* Transcript 9

Lab 2: numbers and strings

Sep 20

Quiz study guide

Sep 22

Quiz 1


Sep 25


First functions, while loops

* Slides 10
* pretty_function.py
* factorial.py

* Slides 11
* mystery_errros.py
* min_max_range.py

* Slides 12
* while_random.py
* guess_number.py

Lab 3: if/else

Sep 27


Sep 29


Oct 02


Graphics, Using Objects

* Slides 13
* circles.py
* Transcript 13

* Slides 14
* box.py
* Transcript 14

* Slides 15
* snow_start.py

Lab 4: while loops/functions

Oct 04

Q2 study guide

Oct 06

Quiz 2


Oct 09


Fruitful Functions

* Slides 16
* build_list.py
* Transcript 16

* Slides 17
* shuffle_list.py

* Slides 18
* Handout 2
* max_circle.py

Lab 5: graphics

Oct 11


Oct 13


Oct 16

Fall Break

Oct 18

Oct 20


Oct 23


File IO, Top-Down Design

* Slides 19
* Handout 3
* colleges_file.py
* cs21_students.py

* Slides 20
* Handout 4
* word_guessTDD.py
* Transcript 20

* Slides 21
* try_except.py
* write_ninjas.py

Lab 6: more functions

Oct 25


Oct 27


Oct 30


More TDD

* Slides 22
* tictactoe_TDD.py

* Slides 23
* ttt_functions.py
* 9letter_words.py

* Slides 24

Lab 7: 3square

Nov 01

Q3 study guide

Nov 03

Quiz 3


Nov 06



* Slides 25
* linear_search.py

* Slides 26
* Handout 5
* binary_search.py

* Slides 27
* searches.py
* dictionary_tests.py

Lab 7: 3square

Nov 08


Nov 10

CR/NC/W Deadline


Nov 13



* Slides 28
* Handout 6

* Slides 29
* pseudocode.py
* sorts.py

* Slides 30
* sort_runtime.py

Lab 8: DH/searching

Nov 15

Q4 study guide

Nov 17

Quiz 4


Nov 20


Classes and Objects

* Slides 31
* pie.py
* student.py
* Transcript 31

* Slides 32
* die.py
* random_gene.py
* Transcript 32

Lab 9: DH/sorting

Nov 22


Nov 24



Nov 27


Classes and Objects

* Slides 33
* random_gene.py
* screen_saver.py
* Transcript 33

* Slides 34
* screen_saver.py
* baseball.py

* Slides 35

Lab 10: classes

Nov 29

Q5 study guide

Dec 01

Quiz 5


Dec 04



* Slides 36
* factorial.py

* Slides 37
* fibonacci.py
* circles.py

* Slides 38
* fractal_tree.py

Lab 11: recursion practice

Dec 06


Dec 08


Dec 11

final exam study guide

Wrap up

* Slides 39

Dec 15

Final Exam: Fri, Dec 15, 7-10pm, Sci Cen 101

Grading Policies

Grades will be weighted as follows:
35%Lab assignments
30%Final Exam
5%Class Participation

Exam and Quizzes

Quizzes will be given at the beginning of class on the days posted on the Schedule. Please look over these dates carefully and contact the professor in advance if you cannot be in class for a quiz.

If you are not present on the day of a quiz, and do not let me know ahead of time that you are missing class, you will receive a zero for that quiz.

There will be one final exam for the semester. Details and dates will be released during the semester. Please read the section on accommodations if you are in need of extra time. You must inform me of accommodations or conflicts at least 2 weeks in advance of the exam.

Lab Policy

This course features weekly lab assignments which are the largest component of your course grade. Lab attendance is required by all students, unless you have already completed and submitted the lab assignment for the week. You must attend the lab session for which you are enrolled:

Weekly Lab Sessions
CS21 A 1:05—2:35pm Tuesdays Mathieson Science Center 240
CS21 B 2:45—4:15pm Tuesdays Mauskop Science Center 240
CS21 C 1:15—2:45pm Wednesdays Mathieson Science Center 240
CS21 D 3:00—4:30pm Wednesdays Wicentowski Science Center 240

Lab assignments will typically be assigned in class at the beginning of the week and will be due before midnight on Saturdays. You are strongly encouraged to start early and to attend the study sessions for extra practice.

You will submit you assignments electronically using the handin21 program. You may submit your assignment multiple times, and a history of previous submission will be saved. You are encouraged to submit your work regularly.

Late Policy:

Late assignments are not accepted and will receive zero credit (see exceptions below). Even if you do not fully complete an assignment, you should submit what you have done to receive partial credit.

The CS labs are open 24 hours a day, 7 days a week for you to use for CS21 lab assignments.

If you feel that you need an extension on an assignment or that you are unable to attend class for two or more meetings due to a medical condition (e.g., extended illness, concussion, hospitalization) or other emergency, you must contact the dean's office and your instructors. Faculty will coordinate with the deans to determine and provide the appropriate accommodations. Note that for illnesses, the College's medical excuse policy, states that you must be seen and diagnosed by the Worth Health Center if you would like them to contact your class dean with corroborating medical information.

Academic Integrity

Academic honesty is required in all your work. Under no circumstances may you hand in work done with (or by) someone else under your own name. Your code should never be shared with anyone; you may not examine or use code belonging to someone else, nor may you let anyone else look at or make a copy of your code. This includes, but is not limited to, obtaining solutions from students who previously took the course or code that can be found online. You may not share solutions after the due date of the assignment.

Discussing ideas and approaches to problems with others on a general level is fine (in fact, we encourage you to discuss general strategies with each other), but you should never read anyone else's code or let anyone else read your code. All code you submit must be your own with the following permissible exceptions: code distributed in class, and code found in the course text book. In these cases, you should always include detailed comments that indicates on which parts of the assignment you received help, and what your sources were.

Failure to abide by these rules constitutes academic dishonesty and will lead to a hearing of the College Judiciary Committee. According to the Faculty Handbook: "Because plagiarism is considered to be so serious a transgression, it is the opinion of the faculty that for the first offense, failure in the course and, as appropriate, suspension for a semester or deprivation of the degree in that year is suitable; for a second offense, the penalty should normally be expulsion." Be aware that we will be routinely running plagiarism detection software on the lab solutions of students from all sections of CS21.

Please contact me if you have any questions about what is permissible in this course.

Student Support

Lauri Courtenay is the CS Department's Academic Support Coordinator. She will be working closely with our student mentors, also known as Ninjas, to help you learn how to program and think like a computer scientist. The CS21 Ninjas will assist me in class and run evening study sessions in the birds' nest (Sci 256). The CS21 Ninjas (student mentors) are: Rye Buckley, Theint Kyaw, Misha Khan, Rachel Diamond, Clarissa Phillips, Anya Chaudhri, Michelle Ma, Christina Holmgren, Tristan Cates, Pravadh Singh, Alexa (Scout) Clark, Brent Jacobs

Study Sessions

You are invited -- and encouraged -- to participate in Ninja evening study sessions to prepare for quizzes, to discuss programming concepts, and to get friendly assistance in working on lab assignments. Our CS mentoring team is dedicated to helping students, who have no prior knowledge of computer science, learn to program in Python while keeping their senses of humor intact. As an added bonus, free snacks will be provided at the sessions. The sessions are held:

Weekly Evening Ninja Sessions
Tuesdays 7—9pm Sci Center 256
Wednesdays 7—10pm Sci Center 256
Fridays 7—9pm Sci Center 256

Accessing the CS labs after hours

You can use your ID to gain access to the computer labs at nights and on the weekends. Just wave your ID over the microprox reader next to the lab doors. When the green light goes on, just push the door to get in (the door knob will not turn). If the green light doesn't go on, then we need to enter your microprox number into the system. Email local-staff@cs.swarthmore.edu (or see Bridget) if you have problems with this. If the building is locked, you can use your ID to enter the door between Martin and Cornell library. For this class, your ID will give you access to the labs in rooms 238, 240, and 256 and Clothier 16.

Academic Accommodations

If you believe that you need accommodations for a disability, please contact the Office of Student Disability Services (Parrish 113W) or email studentdisabilityservices at swarthmore.edu to arrange an appointment to discuss your needs. As appropriate, the Office will issue students with documented disabilities a formal Accommodations Letter. Since accommodations require early planning and are not retroactive, please contact the Office as soon as possible. For details about the accommodations process, visit the Student Disability Service website.

To receive an accommodation for a course activity, you must have an Accommodation Authorization letter from the Office of Student Disability Services and you need to meet with me to work out the details of your accommodation at least one week prior to the activity.

You are also welcome to contact me privately to discuss your academic needs. However, all disability-related accommodations must be arranged through the Office of Student Disability Services.

How to Succeed in CS 21

Programming Style

Programming is not a dry mechanical process, but a form of art. Well written code has an aesthetic appeal while poor form can make other programmers and instructors cringe. Programming assignments will be graded based on style and correctness. Good programming practices usually include many of the following principles:

Also, look over the Python Code Style Guide for more details and some examples of good code style.


This semester we’ll be using Piazza, an online Q&A forum for class discussion, help with labs, clarifications, and announcements that pertain to all sections of cs21. You should have received an email invitation to join CS21 on Piazza. If you didn't, please let us know.

Piazza is meant for questions outside of regular meeting times such as office hours, ninja sessions, class, and lab. Please do not hesitate to ask and answer questions on Piazza, but keep in mind the following guidelines:

  1. Piazza should be used for ALL content and logistics questions outside of class, lab, office hours, and ninja sessions. Please do not email instructors or ninjas with your code or questions about the assignments.
  2. If there is a personal issue that relates only to you, please email your instructor.
  3. We encourage non-anonymous posts, but you may post anonymously (to your classmates, not the instructors).
  4. Do NOT post long blocks of code on Piazza - if you can distill the problem to 1-2 lines of code and an error message, that’s fine, but try to avoid giving out key components of your work.
  5. By the same token, when answering a question, try to give some guiding help but do not post code fixes or explicit solutions to the problem.
  6. Posting on Piazza counts toward your participation grade, both asking and answering!

Links that are related to the course may be posted here. If you have suggestions for links, let me know.

Algorithms to Live By...
How To Think Like a Computer Scientist: Python for Software Design
Python Tutor
Python style guide From Prof. Tia Newhall
Using Unix
Basic Unix Commands
Graphics reference
Dive Into Python
(A Semi-Official) Python FAQ Zone
Python Documentation (Note: we are using v3.5)
Remote access with atom
Vi Quick Reference
emacs beginner's guide